For a variety of reasons, it is often necessary to measure the amount of aerosol particulate matter in a moving fluid stream. Four examples of such measurement requirements are given below. These examples are merely illustrations.
The concentration of particulate matter in industrial smokestacks is frequently measured with apparatus which separates aerosol particles that are not inhalable into the thoracic region of the human respiratory system from those that are. The latter category of particles is associated with sizes less than or equal to 10 micrometers aerodynamic diameter and is referred to as PM-10 aerosol. Fractionation of the aerosol size distribution is accomplished by drawing stack gas into a probe and then through a device such as a cyclone separator. To maintain a consistency of fractionation characteristics, the air flow rate through the separator must be held constant in spite of needs for varying the inlet velocity through the probe in order to satisfy isokinetic requirements. PM-10 particulate matter which passes through the stack is measured to determine compliance with federal, state and local regulations.
An example of continuously monitoring a moving gas stream for particulate matter is that associated with a ventilation system for a clean room. Such rooms are used for assembling precision electronic and mechanical devices and for medical and biological purposes. The air to be supplied to a clean room is generally filtered mechanically. It is important that the air is then continuously monitored and analyzed for concentrations of airborne particles of certain sizes to insure that the level of contamination in the room is compatible with usage requirements.
The useful life of gas turbine engines is dependent on the amount of atmospheric dust drawn in with the combustion air. In the case of small engines as in helicopters, the cost of monitoring the dust in the inlet air is not justified. In large industrial applications, a defective filter can destroy a multimillion dollar engine. It is important to periodically or continuously monitor the inlet air for particulate contamination.
A fourth instance of the importance of measuring particulate matter in a gas stream is in the field of processing and handling of nuclear materials waste. In one application, nuclear waste is to be sealed in drums and stored in an underground salt mine. During the time period (approximately 25 years) that the waste will be received at the mine, ventilation air is drawn down mine shafts and exhausted back to the surface through a central shaft. If radioactivity is inadvertently released, continuous air sampling devices would need to activate alarms and ventilation air controls to protect workers and prevent release of radioactive particles to the environment.
The known method of taking samples from a moving fluid stream is to introduce a probe into that stream and withdraw a fluid sample. The sample is then analyzed to determine the characteristics of particulate matter. The physical presence of the probe in a moving gas stream disturbs the gas flow in the vicinity of the probe. As the gas is forced past or into the probe, the normally parallel flow paths of the gas are curved or distorted. Particulate matter carried along by the gas stream is subjected to inertial forces which tend to cause the particles to continue in a straight line. Because these inertial forces are directly proportional to the particle mass, larger and/or more dense particles tend to deviate from the curving gas flow less than smaller and/or less dense particles. This could cause a sample to have a disproportionately large or small particulate concentration relative to the stream being sampled depending on how the flow is disturbed and what size and type of particles are in that stream.
Previous attempts to solve this problem have been by the use of isokinetic probes. An isokinetic probe is operated so that the fluid velocity (V) inside the mouth of the probe is the same as the temporal mean free stream velocity (U) upstream of the probe. The term isokinetic is derived from the fact that the specific kinetic energy of the free stream is the same as that of the sample entering the probe. Isokinetic sampling is distinguished from sub-isokinetic sampling, in which the velocity of the sample in the probe (V) is less than the mean velocity of the free stream (U) and supra-isokinetic sampling in which V is greater than U. Isokinetic operation (V=U) causes the least amount of flow disturbance.
However, continuous isokinetic operation of a probe is difficult to obtain in actual applications because a change in the velocity of the free stream must be accompanied by a change in velocity of the probe sample. It is apparent that for continuous monitoring a complicated sensing and control system is required to maintain the fluid velocity in the probe the same as that of a varying free stream. Also, if the sample is being fed to a fractionator for analysis, that fractionator will require a constant flow rate. If the flow rate must be maintained constant to the fractionator but varied through the probe, an even more complex control or sampling system is required.
An additional problem of operating an isokinetic probe is that of wall losses due to either turbulent deposition or anisokinetic effects. In the case of turbulent deposition, large diameter probes and low velocities are preferred. The larger the probe diameter, the smaller the ratio of particle stopping distance to probe diameter and less the chance for material to be driven from the sampled stream to the wall. Lower velocities are preferred because the inertial effects are reduced as velocity V is decreased. However, in isokinetic sampling the velocity V is fixed by U and the probe diameter is fixed by the combination of flow rate requirements and the velocity V.